Oracle: allow multiple feed values from inner calls

[lemunozm]

Description

This PR shows an example of an implementation to allow calling feed_value() multiple times internally, avoiding the AlreadyFeeded error without being a breaking change and without effecting to the extrinsic behavior

Related issue: #919

[xlc]

I think this works. Just need some tests and docs.

[codecov-commenter]

Codecov Report

Merging #920 (33afe6f) into master (055e7db) will increase coverage by 0.00%.
The diff coverage is 90.90%.

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@@           Coverage Diff           @@
##           master     #920   +/-   ##
=======================================
  Coverage   78.21%   78.21%           
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  Files         105      105           
  Lines       10621    10623    +2     
=======================================
+ Hits         8307     8309    +2     
  Misses       2314     2314           

Impacted Files	Coverage Δ
oracle/src/lib.rs	75.00% <90.90%> (+0.92%)	⬆️

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[lemunozm]

Would you agree to add to this PR a minor breaking change to allow passing Option<AccountId> to feed_value()? Would be much more ergonomic to have a "default" member easy to use.

[xlc]

I think it is ok.
It is a breaking change which isn't ideal, but otherwise the code itself won't be ideal as well. It is not worth to sacrifice code maintainability for such trivial breaking change.

[lemunozm]

I think this is ready for review. I've also added the test part related to these changes.