feat(pay): open payments support

[kincaidoneil]

• Resolve and pay Open Payments invoices with invoiceUrl option
• Add docs, schema, and validation for invoices
• Query Open Payments account in parallel with SPSP query
• Simplify some config options
• Improve test coverage to 100%
• Fix probed exchange rate mistakenly reset
• Fix sending u64 probe packet, rate would incorrectly be 1:1
• Update deps for all packages, miscellaneous fixes

[codecov]

Codecov Report

Merging #56 into master will increase coverage by 0.63%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   90.04%   90.68%   +0.63%     
==========================================
  Files          33       32       -1     
  Lines        2201     2297      +96     
  Branches      380      400      +20     
==========================================
+ Hits         1982     2083     +101     
+ Misses        219      214       -5     

Flag	Coverage Δ
#connection_tag_utils	100.00% <ø> (ø)
#ilp_logger	91.66% <ø> (ø)
#ilp_packet	63.84% <ø> (ø)
#ilp_plugin	100.00% <ø> (ø)
#ilp_protocol_ccp	98.78% <ø> (ø)
#ilp_protocol_ildcp	100.00% <ø> (ø)
#ilp_spsp_payout	14.39% <ø> (ø)
#oer_utils	99.37% <ø> (ø)
#pay	100.00% <100.00%> (+0.47%)	⬆️

Impacted Files	Coverage Δ
packages/pay/src/controllers/failure.ts	100.00% <ø> (ø)
packages/pay/src/connection.ts	100.00% <100.00%> (ø)
packages/pay/src/controllers/amount.ts	100.00% <100.00%> (ø)
packages/pay/src/controllers/asset-details.ts	100.00% <100.00%> (ø)
packages/pay/src/controllers/exchange-rate.ts	100.00% <100.00%> (ø)
packages/pay/src/controllers/max-packet.ts	100.00% <100.00%> (ø)
packages/pay/src/controllers/rate-probe.ts	100.00% <100.00%> (ø)
packages/pay/src/controllers/sequence.ts	100.00% <100.00%> (+14.28%)	⬆️
packages/pay/src/index.ts	100.00% <100.00%> (ø)
packages/pay/src/open-payments.ts	100.00% <100.00%> (ø)
... and 4 more

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[matdehaast]

LGTM