[fix](iceberg) fix logic of getting namespaces

[morningman]

What problem does this PR solve?

Followup #56415

Problem Summary:

1. The previous getNamespace logic is wrong, we should split the dbName by . to create namespaces.

2. Allow not specify oauth.uri of iceberg rest catalog, to follow the new spec of IRC

   So we can connect Snowflake open catalog like this:

   CREATE CATALOG ice PROPERTIES (
       'type' = 'iceberg',
       'warehouse' = 'yy_external_catalog3',
       'iceberg.catalog.type' = 'rest',
       'iceberg.rest.uri' = 'https://xxx.snowflakecomputing.com/polaris/api/catalog',
       'iceberg.rest.security.type' = 'oauth2',
       'iceberg.rest.oauth2.credential' = 'id:secrete,
       'iceberg.rest.oauth2.scope' = 'PRINCIPAL_ROLE:yy_sn_principal_role',
       'iceberg.rest.nested-namespace-enabled' = 'true',
       's3.endpoint' = 'https://s3.us-west-2.amazonaws.com',
       's3.region' = 'us-west-2',
       'iceberg.rest.nested-namespace-enabled' = 'true'
   );
   

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
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4. Which code was refactored and why was this part of the code refactored?
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[morningman]

run buildall

[doris-robot]

ClickBench: Total hot run time: 30.54 s

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/clickbench-tools
ClickBench test result on commit 288a734f90c529753e30297def9d1839e3def750, data reload: false

query1	0.05	0.05	0.05
query2	0.09	0.05	0.06
query3	0.25	0.08	0.08
query4	1.60	0.12	0.11
query5	0.27	0.27	0.25
query6	1.17	0.66	0.64
query7	0.04	0.03	0.03
query8	0.06	0.05	0.05
query9	0.62	0.54	0.52
query10	0.60	0.59	0.58
query11	0.16	0.11	0.12
query12	0.15	0.12	0.12
query13	0.63	0.61	0.63
query14	1.03	1.05	1.02
query15	0.90	0.84	0.86
query16	0.40	0.39	0.40
query17	1.04	1.05	1.05
query18	0.21	0.20	0.20
query19	1.99	1.83	1.86
query20	0.02	0.02	0.01
query21	15.43	0.92	0.56
query22	0.78	1.21	0.73
query23	14.81	1.38	0.64
query24	6.76	0.87	1.09
query25	0.46	0.23	0.07
query26	0.59	0.17	0.13
query27	0.07	0.05	0.06
query28	10.20	1.40	0.92
query29	12.58	4.03	3.38
query30	0.28	0.13	0.11
query31	2.83	0.61	0.39
query32	3.23	0.57	0.48
query33	3.09	3.24	3.04
query34	16.17	5.44	4.81
query35	4.94	4.93	4.94
query36	0.71	0.52	0.50
query37	0.10	0.08	0.07
query38	0.07	0.05	0.04
query39	0.04	0.03	0.03
query40	0.18	0.17	0.16
query41	0.08	0.03	0.04
query42	0.04	0.03	0.03
query43	0.05	0.04	0.03
Total cold run time: 104.77 s
Total hot run time: 30.54 s

[hello-stephen]

FE UT Coverage Report

Increment line coverage 76.92% (10/13) 🎉
Increment coverage report
Complete coverage report

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 7.69% (1/13) 🎉
Increment coverage report
Complete coverage report

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.