add CODECOV_TOKEN secret

[jGaboardi]

@carsonfarmer I think you may have to "turn on" codecov for the report from CI, (see failed report upload here). You should be able to access via [https://app.codecov.io/gh/carsonfarmer/fastpair/new] and follow the instructions for adding a CODECOV_TOKEN repository secret. This is low on the list of criticalities, so no rush at all (but also does only take 5 seconds).

xref #20

[carsonfarmer]

Done. Should be good to go. Please reopen if I have not done it correctly.

[jGaboardi]

Done. Should be good to go. Please reopen if I have not done it correctly.

Yep, can confirm upload in #27 --> https://github.com/carsonfarmer/fastpair/actions/runs/9275053234/job/25518848417?pr=27#step:7:58

I'll add in a codecov.yaml for more customized reporting options later.