Add missing license metadata to mcp-macro

[niebayes]

The Cargo.toml manifest file of the mcp-macro crate does not contain a license metadata, which makes cargo-deny fails.

error[unlicensed]: mcp-macros = 1.0.7 is unlicensed
  ┌─ git+https://github.com/modelcontextprotocol/rust-sdk.git?rev=4597d1f047416a2f51e35372bc35b02aae2ec8b0#mcp-macros@1.0.7:2:9
  │
2 │ name = "mcp-macros"
  │         ━━━━━━━━━━ a valid license expression could not be retrieved for the crate
3 │ version = "1.0.7"
4 │ license = ""
  │            ─ license expression was not specified

[niebayes]

@alexhancock PTAL