Replace Guardian Key with abstracted Guardian Signer

[pleasew8t]

This pull request introduces the GuardianSigner interface, defined in the guardiansigner package, which is meant to replace directly using the guardian key (private key) for data signing. In doing so, it becomes easier to introduce alternative signing mechanisms without too much modification of code outside of the new guardiansigner package. Additional signing mechanisms include HSMs or KMS's (such as AWS or GCP).

The changes made to the repository are summarised as follows:

• Introduction of the new guardiansigner package, which includes a FileSigner implementation that works with the current guardian key.
• Addition of the --guardianSignerUri commandline argument. Node operators can still make use of guardianKeyPath, as support is provided to translate the path to the appropriate guardianSignerUri. As additional signer implementations are introduced, more URI schemes will be added. But for now only file:// is supported, loading a private key from disk.
• Replace data signing that used the guardian key with signing using a GuardianSigner.

Notable Code Change

One change in the PR that we are not yet certain about is the use of AddSignature defined in sdk/vaa/structs.go. The changes in node/pkg/adminrpc/adminserver.go removed the use of AddSignature and adds the signature manually, to make use of a GuardianSigner. There were some alternative ideas surrounding this:

An alternative idea was to make AddSignature accept a GuardianSigner instead of a private key. This did not seem right, as it would create a dependency on the node packages that seems unneccessary, and also modify what is defined as an SDK, potentially breaking functionality for other projects that might make use of the SDK.

Additional comment by @johnsaigle that is also worth considering: #4120 (comment)

[banescusebi]

Just one edge case and some small nits

[johnsaigle]

Left some minor comments related to naming and clarity. Overall the implementation looks good

[johnsaigle]

AddSignature in sdk/vaa/structs.go is now unused outside of tests. Should we mark it as deprecated? Are there any other related functions that should be deprecated now? Or we could go further and delete the code if the intention is that this new mechanism should totally replace the old one.

Another approach would be to rewrite this function to use the new Guardian signer though it would be a breaking change to modify the function signature.

[djb15]

Very cool!

[SEJeff]

All of my suggestions here are in general nits or things that make future maintenance a bit nicer. This is a nice clean implementation from all I see here. Nicely done sir! 🥳