[FEATURE] Can't set "X-WaitFor" header

[sirdeggen]

Migrated from bsv-blockchain/ts-sdk#283

ARC interprets a request header X-WaitFor.

https://bitcoin-sv.github.io/arc/api#/Arc/POST%20transaction

Setting this header will instruct ARC on which status to wait for before it will return the response.

Currently it seems not to be possible to set this header with ts-sdk or maybe I'm doing something wrong.

TypeError: Cannot set properties of undefined (setting 'X-WaitFor')

It would be quite useful to be able to set this header using ts-sdk

[ty-everett]

Completed in PR #320. ARC requests now accept caller-supplied headers, including X-WaitFor, with regression coverage and passing SDK tests.