feat: Allow users to configure logging level of bitcoin adapter

[ielashi]

Description

The bitcoin adapter's logging can be very verbose if debug logging is enabled, making it difficult to make sense of what's going on. On the other hand, these logs are useful for triaging problems.

To get the best of both worlds, this commit adds support for an additional configuration option in dfx.json:

  "defaults": {
    "bitcoin": {
      "enabled": true,
      "nodes": ["127.0.0.1:18444"],
      "log_level": "info" <------- users can now configure the log level
    },

By default, a log level of "info" is used, which is relatively quiet. Users can change it to "debug" for more verbose logging.

How Has This Been Tested?

I ran dfx start with various configurations of dfx.json and verified from the logs that the bitcoin adapter gets initialized with the expected log level.

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[viviveevee]

LGTM