fix!: store headers in BTreeMap instead of HashMap

[ilbertt]

Description

BREAKING: Use BTreeMap instead of HashMap for headers to guarantee deterministic ordering.

How Has This Been Tested?

Same tests should still pass.

Checklist:

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

[sesi200]

HashMap and BTreeMap seralize to the same thing with Candid, so this will not break the stable state deserialization

[ilbertt]

HashMap and BTreeMap seralize to the same thing with Candid, so this will not break the stable state deserialization

I've added a couple of tests to check this