feat(agent crate): adds remote mcp support in agent crate

[dingfeli]

Issue #, if available:

Description of changes:

• Adds support to http transport
• Slight architectural changes where the instantiation of McpServerActor is given a mpsc::Sender<McpServerActorEvent> as opposed to each McpServerActor having to spawn their own pair. This is because this allows us to poll a single mpsc::Receiver<McpServerActorEvent>, as compared to before, where we have to call recv on all of the McpServerActor and putting them all a collection impl Future.

Changes since review 1:

• adds method to see if mcp config is disabled
• changes mpsc to broadcast for mcp_main_loop_to_handle_server_event_rx
• expands path using provided util function
• uses event buffer for mcp events
• relocates server event sender to main loop
• restores use of oneshot in server launch
• propagates appropriate messages to prevent leaky abstractions

Tests
Test case: cached credentials, making tool calls (this was ran before the decoupling of Model from the agent crate):

➜  amazon-q-developer-cli jj:(dingfeli/adds-http-support-agent-crate) ✗ RUST_LOG=trace cargo run -p agent -- run --agent=test_test --dangerously-trust-all-tools "what notion docs do i have"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.13s
     Running `target/debug/agent run --agent=test_test --dangerously-trust-all-tools 'what notion docs do i have'`
I'll search your Notion workspace to see what documents you have available.
{
  "toolUseId": "tooluse_e6OugG3yTeWBD4ZDc_peSw",
  "name": "notion-search",
  "input": {
    "__tool_use_purpose": "Search the user's Notion workspace to find all available documents",
    "query": "*",
    "query_type": "internal"
  }
}
Based on the search results, here are the Notion documents you have:

**Task Management:**
- Weekly To-do List
- My tasks

**Getting Started:**
- Welcome to Notion! (onboarding page)

**Other:**
- Home views

You have a mix of creative content, financial tracking documents, and productivity tools. Most were created about a month ago, with the Steam Deck poem being more recent (11 days ago).%

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