Expose StoredKey encryption parameters

[optout21]

Description

Expose StoredKey encryption parameters, through C interface, as Json string.

StoredKey unit tests.

Types of changes

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

Checklist

• Prefix PR title with [WIP] if necessary.
• Add tests to cover changes as needed.
• Update documentation as needed.
• I have read the guidelines for adding a new blockchain
• If there is a related Issue, mention it in the description (e.g. Fixes #<issue_number> ).

[optout21]

Encryption parameters can be controlled from outside:

• There is an enum with values Default, Weak and Standard https://github.com/trustwallet/wallet-core/pull/1766/files#diff-ff71e2e6bd79b5044dd5c8c11a0ae08cf4df0413d51ecb8f9b4d074bcff5e590
• When creating StoredKey wallet storage, level can be specified
• Weak level is with 4096, used currently
• Standard level is with 262k.
• On macos, with debug build, creation with Weak takes ~1 sec, with Standard ~8 secs.

[vikmeup]

Weak level is with 4096, used currently

let's change it to 10K as it's usually a default option in many places

[optout21]

Weak level is with 4096, used currently

let's change it to 10K as it's usually a default option in many places

I have added a new level, with 16k (power of 2); #1801 .