allow customizing template name in `valueOr` / `errorOr`

[etan-status]

Currently, valueOr / errorOr support is degraded when used from a generic or template context, due to naming collision of the injected template and different precedence being applied in such contexts. To still allow using the functionality as expected, provide overloads that allow customizing the field names.

https://github.com/status-im/nim-stew/issues/161#issuecomment-1397121386

[etan-status]

Usage example:

proc decodeLightClientObjectHttpResponse[T: SomeForkedLightClientObject](
    resp: RestHttpResponseRef,
    data: Future[seq[byte]],
    cfg: RuntimeConfig,
    x: typedesc[T]): Future[T] {.async.} =
  let
    consensusFork = decodeEthConsensusVersion(
      resp.headers.getString("eth-consensus-version")).valueOr(resError):
        raiseRestDecodingBytesError(resError)
    mediaType = decodeMediaType(resp.contentType).valueOr(resError):
      raise newException(RestError, $resError)
    obj =
      if mediaType == OctetStreamMediaType:
        decodeBytes(T, await data, resp.contentType).valueOr(resError):
          raiseRestDecodingBytesError(resError)
      elif mediaType == ApplicationJsonMediaType:
        withLcDataFork(lcDataForkAtConsensusFork(consensusFork)):
          when lcDataFork > LightClientDataFork.None:
            var obj = T(kind: lcDataFork)
            obj.forky(lcDataFork) = SSZ.decode(
              await data, T.Forky(lcDataFork))
            obj
          else:
            raiseRestDecodingBytesError(
              cstring("Unsupported fork: " & $consensusFork))
      else:
        raise newException(RestError, "Unsupported content-type")
    objectFork = withForkyObject(obj):
      when lcDataFork > LightClientDataFork.None:
        cfg.consensusForkAtEpoch(forkyObject.contextEpoch)
      else:
        raiseRestDecodingBytesError("Invalid data")
  if consensusFork != objectFork:
    raiseRestDecodingBytesError(cstring("Inconsistent forks" &
      " (header: " & $consensusFork & ", data: " & $objectFork & ")"))
  return obj

[etan-status]

From nimbus-eth2 PM discussion 2023-03-22 it was suggested to work around this limitation in nimbus-eth2 instead of nim-stew.