Abigious extra one byte needed when parsing StorageMap keys in get_oracle_headings client function

[ameba23]

See disussion in PR: #1170 (comment)

There is some ambiguity in how StorageMap keys are composed. It appears the are the concatenation of storage address (32 bytes), blake2 hash of key (16 bytes), and key itself (variable length). But there is an extra 1 byte needed.

@HCastano suggested this might be related to theQueryKind being set to OptionQuery.

We should check the storagemap source code and confirm - just in case this is not something which will reliably always be present, and our code breaks sometime.

See also the client function we use to read the key value pairs: subxt::storage::Storage::iter