fix: deserialize all trigger metadata as strings

[kate-goldenring]

When updating to latest spin dependencies, we changed trigger metadata to no longer be strings: #40

Right now, the trigger fails to run with the errors:

Error: metadata error: invalid metadata value for "mqtt": Error("invalid type: string \"30\", expected u64", line: 0, column: 0)

And

Error: json error: invalid type: string "1", expected i32

Caused by:
    invalid type: string "1", expected i32

This PR resolves the error by making all deserialized metadata strings.

[kate-goldenring]

This provides motivation to add more tests to the repo

[rylev]

Whoops! Thanks for fixing this.