Epic: Move/port identified `yapapi` bits to `golem-core`

[approxit]

After #54 and #55, we can finally cut out yapapi dependencies to get golem-core into state, with no python-golem-related stuff at the lower layer, just directly yagna.

Note: check both yapapi's version for each bit (^0.9.3 and newest)
Note: Ensure that examples are working

Bits

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1. Port/reimplemnt Payload along with yapapi.payload.vm and other yapapi.payload artifacts in golem-core: #77
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2. Port yapapi.GftpProvider to golem-core #76
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3. Port/reimplement rest.Configuration #75
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4. Port yapapi.DemandBuilder and yapapi.props to golem-core #74
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5. Copy minor bits from yapapi to golem-core #73
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6. Add black and isort to checks #93
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7. Refactor modules structure in golem-core-python #92
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8. Remove yapapi from pyproject.toml #91
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9. Rewrite README

[approxit]

We're waiting to conclude results of #54, to actually mark this issue as ready.