feat: stewardship for feeds

[significance]

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Context

it's not possible to validate that (indexed) feed chunks are available in the network. it would be good to be able to check that feed chunks are available up to a certain index.

something like:

curl http://localhost:1633/stewardship/feed/ae5b5991481f3e14c60e082d71e43dd3ffe7cdac326b844cda412e9986e9eb4d?index=10

[{index: "0", "isRetrievable":true},...,{index: "0", "isRetrievable":true}]

it would be best if all the chunks are checked independent of each other to highlight situations where there are chunks missing from the middle of the sequence.

[significance]

there was some discussion that the best approach may be to polyfil our needs using js for now, but the issue should probably remain as it represents one of the gaps in the bee api that should be filled when priorities allow