fix: fix SegWit tx size estimation (in vByte) and gas fee calculation

[ws4charlie]

Description

1. use estimated SegWit tx size for Bitcoin gas fee calculation
2. estimate SegWit tx size in uinit of vByte
   see doc: https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki#transaction-size-calculations
3. increase depositor fee rate from 5 sats/vB --> 20 sats/vB

Closes: 1661
Closes: 1667

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Include instructions and any relevant details so others can reproduce.

• Tested CCTX in localnet
• Tested in development environment
• Go unit tests
• Go integration tests
• Tested via GitHub Actions

Checklist:

• I have added unit tests that prove my fix feature works

[github-actions]

!!!WARNING!!!
nosec detected in the following files: zetaclient/btc_signer.go, zetaclient/btc_signer_test.go, zetaclient/utils.go

Be very careful about using #nosec in code. It can be a quick way to suppress security warnings and move forward with development, it should be employed with caution. Suppressing warnings with #nosec can hide potentially serious vulnerabilities. Only use #nosec when you're absolutely certain that the security issue is either a false positive or has been mitigated in another way.

Only suppress a single rule (or a specific set of rules) within a section of code, while continuing to scan for other problems. To do this, you can list the rule(s) to be suppressed within the #nosec annotation, e.g: /* #nosec G401 */ or //#nosec G201 G202 G203
Broad #nosec annotations should be avoided, as they can hide other vulnerabilities. The CI will block you from merging this PR until you remove #nosec annotations that do not target specific rules.

Pay extra attention to the way #nosec is being used in the files listed above.

[kevinssgh]

Looks good

[kingpinXD]

lgtm , minor comments