refactor: simplify IDataProvider

[guidanoli]

• The IDataProvider implementation should be aware of epoch boundaries, and receive input indices relative to the epoch, to facilitate usage by tournament contracts
• namespace is not necessary because, as of now, it's just a constant
• extra is always input in the current stage
• Block numbers are not necessary, because input indices delimit epoch boundaries already
• Without thinking in terms of block numbers, we don't need to decode input blobs

[GCdePaula]

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