Improve ergonomics of prompt and sampling message initialization

[mattt]

Follow-up to #119

This PR improves the ergonomics for constructing prompt and sampling messages in the following ways:

• Replace Message(role:content:) initializers with .user(_ content:) and .assistant(_ content:) factory methods
• Making Message.Content expressible by string literals (with support for interpolation).

Before

Prompt Messages:

let messages = [
    Prompt.Message(role: .user, content: .text(text: "Tell me about yourself")),
    Prompt.Message(role: .assistant, content: .text(text: "I'm a software engineer with 5 years of experience"))
]

// Interview scenario
let candidateName = "Bob"
let position = "Data Scientist"
let interviewMessage = Prompt.Message(
    role: .assistant,
    content: .text(text: "Welcome \(candidateName)! Tell me about your experience as a \(position)")
)

Sampling Messages:

let messages = [
    Sampling.Message(role: .user, content: .text("What's the weather like?")),
    Sampling.Message(role: .assistant, content: .text("I'd be happy to help you check the weather!"))
]


// Mixed content types
let conversation = [
    Sampling.Message(role: .user, content: "Can you analyze this image?"),
    Sampling.Message(role: .assistant, content: .image(data: "base64data", mimeType: "image/png")),
    Sampling.Message(role: .user, content: "What does it show?")
]

After

Prompt Messages:

let messages: [Prompt.Message] = [
    .user("Tell me about yourself"),
    .assistant("I'm a software engineer with 5 years of experience")
]

// Interview scenario with string interpolation
let candidateName = "Bob"
let position = "Data Scientist"
let interviewMessage: Prompt.Message = .assistant("Welcome \(candidateName)! Tell me about your experience as a \(position)")

Sampling Messages:

let messages: [Sampling.Message] = [
    .user("What's the weather like?"),
    .assistant("I'd be happy to help you check the weather!")
]

// Mixed content types
let conversation: [Sampling.Message] = [
    .user("Can you analyze this image?"),
    .assistant(.image(data: "base64data", mimeType: "image/png")),
    .user("What does it show?")
]

For backwards compatibility, the existing Message initializers are deprecated. In the future, they'll be made internal.