Refine and Document `ErgoGenerators.extensionKvGen()`

[stenolog]

• re https://github.com/ergoplatform/ergo/issues/2114

The sporadic failure of test ErgoStateContextSpec (issue #2114) uncovered problems with ErgoGenerators.extensionKvGen()

Relevant Code

• see https://github.com/ergoplatform/ergo/pull/2116

Tasks

• [x] https://github.com/ergoplatform/ergo/issues/2117
• [ ] add in-code documentation to ErgoGenerators.extensionKvGen()
• [ ] revise code to handle the problems uncovered within #2114
  • Possibly introduce extensionKvGenValidated()

[kushti]

There is no problem with generator , for keyspace starting with 0 byte, length of values should be of 4 bytes, otherwise, tests with fail on extension section parsing due to

def parseExtension(h: Height, extension: Extension): Try[Parameters] = Try {
    val paramsTable = extension.fields.flatMap { case (k, v) =>
      require(k.length == 2, s"Wrong key during parameters parsing in extension: $extension")
      if (k(0) == Extension.SystemParametersPrefix && k(1) != SoftForkDisablingRules) {
        require(v.length == 4, s"Wrong value during parameters parsing in extension: $extension")

and there's need to document (in EIP) key spaces and rules for them, not generator