Update for Ethereum Keys

[wilwade]

Frequency now supports Ethereum keys, and we want to expand the support in SIWF to include Ethereum as well.

• Update Signed Request Spec Docs for Eth #331
• Update Response Spec Docs for Eth #332
• Update Signed Request Generator to support window.ethereum #333
• Update Docs Examples to show both Eth and sr25519 #334
• Update Data Structure Reference Examples to show both Eth and sr25519 #335
• Update libraries/siwf(v2) to support Eth v2.1 #336
• Update tools/mock-siwf to support Eth #337
• Update tools/js-example to support Eth #338