feat: support current and next version of Stellar protocol XDR

[nebolsin]

Note: this is work in progress.

• extract XDR generated wrappers into separate stellar-xdr gem
• rake task to regenerate the wrappers based on the definitions from stellar/stellar-xdr repo

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[coveralls-official]

Pull Request Test Coverage Report for Build 6271930396

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.518%

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Totals
Change from base Build 6269750352:	0.0%
Covered Lines:	5169
Relevant Lines:	5346

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💛 - Coveralls

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: -1.00%)	∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (1db3a01)	2849	2672	93.79%
Head commit (64a465d)	2849 (+0)	2672 (+0)	93.79% (+0.00%)

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 (#309)	0	0	∅ (not applicable)

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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