feat(soroban): support protocol 20

[nebolsin]

Fixes #347.

• update XDR definitions for protocol 20
• implement contract representation in StrKey
• InvokeHostFunction operation
• BumpExpirationFootprint operation
• RestoreFootprint operation

[coveralls-official]

Pull Request Test Coverage Report for Build 7606653998

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.6%) to 87.694%

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Totals
Change from base Build 7606647840:	2.6%
Covered Lines:	6615
Relevant Lines:	7466

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💛 - Coveralls

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +2.53% (target: -1.00%)	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (7dbb97f)	6108	5257	86.07%
Head commit (cf22f80)	7466 (+1358)	6615 (+1358)	88.60% (+2.53%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#357)	45	45	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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