[WIP] Add support for longer RSA keys longer than 2048, init ECDSA support

[tomaszkrzyzanowski]

Closes #20

as @cipherboy suggested I have created the code to support longer RSA and added support for ECDSA keys

Looking forward to suggestions as I'm not very proficient in Go yet :)

• RSA part was tested (load key, sign) with 4096 bit keys.
• ECDSA was tested with ecdsa-p521 curve (load only yet)

[tomaszkrzyzanowski]

@cipherboy thanks for all the suggestions, the idea behind merging checkKeyType and wrapPrivateKey looks much more obvious right now 😅

I'm going to test out the EC keys during the weekend and leave the feedback if they are working fine or require some more work :)

[tomaszkrzyzanowski]

@cipherboy I have tested yesterday the EC key-based signatures, and it doesn't work.

It fails to verify the Vault Transit's signature on the notation cli side right after signature creation - the plugin output is failing with

Error: generated signature failed verification: signature is invalid. Error: crypto/ecdsa: verification error

But as for now I have no clue, why Vault would respond with a broken signature - probably sth in invocation of "SignWithTransit()" - I assumed that "signature_algorithm" should be empty for ECDSA (as Vault transit api doc say nothing about EC for that field)

So seems like I'm stuck right now with this, and I'm not sure how I should approach this to solve this, or maybe remove the EC parts and deliver longer RSA only for now

[tomaszkrzyzanowski]

I have longer break, but I finally have time to tidy this up

I'm going to provide smaller PR just for RSA longer keys, as it blocks me to use the plugin it in project

[tomaszkrzyzanowski]

Closing as #23 contains cleaned-up version