* Updated the `ecdsa.ParseDERSignature` to include the MaxSigLen constant, which defines the maximum length of a DER encoded signature.
* The MaxSigLen constant is set to 72 bytes, as per the explanation in the comment.
* A new test case has been added to test the functionality with a long signature.
* The test case checks if signatures longer than the specified maximum length are properly handled.
* The test ensures that signatures exceeding the maximum length are correctly identified as invalid.
In this commit, we create a new package to house the ECDSA-specific
logic in the new `btcec/v2` pacakge. Thsi c hange is meant to mirror the
structure of the `dcrec` package, as we'll soon slot in our own custom
BIP-340 implementation.
