Add transaction signer

[otherview]

Description

There seems to be a lot of repetition around the build and the signing of transactions.
Most recognizable by the fact that different tests have built different helpers to make these operations faster.

This PR enhances the tx package to help in this case:

• Introduces the following methods:

MustSignTx(tx *Transaction, pk *ecdsa.PrivateKey) *Transaction

MustCustomSignTx(sign SignFunction, tx *Transaction, pk *ecdsa.PrivateKey) *Transaction

SignTx(tx *Transaction, pk *ecdsa.PrivateKey) (*Transaction, error)

CustomSignTx(sign SignFunction, tx *Transaction, pk *ecdsa.PrivateKey) (*Transaction, error)

DelegatorSignTx(tx *Transaction, pk *ecdsa.PrivateKey) (*Transaction, error)

CustomDelegatorSignTx(sign SignFunction, tx *Transaction, pk *ecdsa.PrivateKey) (*Transaction, error)

Fixes # (issue)

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 44.82759% with 16 lines in your changes missing coverage. Please review.

Project coverage is 62.63%. Comparing base (48cead2) to head (5c15945).

Files with missing lines	Patch %	Lines
tx/signer.go	40.74%	13 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #850      +/-   ##
==========================================
- Coverage   62.65%   62.63%   -0.03%     
==========================================
  Files         208      209       +1     
  Lines       18926    18952      +26     
==========================================
+ Hits        11858    11870      +12     
- Misses       5970     5982      +12     
- Partials     1098     1100       +2     

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