[fix] #4287: use algorithm in PrivateKey serialized form

[0x009922]

Description

Aligns serialized form of PrivateKey with its representation in Rust, i.e. rename digest_function to algorithm.

This change might cause compatibility issues, but let's consider them as a part of the overall breaking configuration change: #2585

Linked issue

Closes #4287

[github-actions]

@BAStos525

[coveralls]

Pull Request Test Coverage Report for Build 8056281809

Details

• 0 of 73 (100.0%) changed or added relevant lines in 4 files are covered.
• 3125 unchanged lines in 55 files lost coverage.
• Overall coverage decreased (-0.3%) to 56.532%

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Files with Coverage Reduction	New Missed Lines	%
core/src/sumeragi/network_topology.rs	1	98.78%
tools/kagami/src/crypto.rs	1	12.12%
data_model/src/metadata.rs	2	78.87%
primitives/src/unique_vec.rs	2	91.24%
config/src/kura.rs	3	80.0%
config/src/logger.rs	5	72.73%
logger/src/lib.rs	5	94.12%
tools/kagami/src/main.rs	5	0.0%
telemetry/src/dev.rs	7	0.0%
core/src/smartcontracts/isi/mod.rs	8	76.4%

Totals
Change from base Build 7884695009:	-0.3%
Covered Lines:	22454
Relevant Lines:	39719

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💛 - Coveralls