Multiple eligibility checks (conditions) for shipping method

[dlhck]

Migrated from #475

Is your feature request related to a problem? Please describe.
Currently one shipping method can apply only one eligiblity checker function. In case that multiple conditions apply we need to write in code completely new eligibility checker that combines all needed checks.

Describe the solution you'd like
Analogously to "Promotion" where we can add multiple conditions we would like to be able to add multiple eligibility checkers to shipping (and in case of upcoming implementation of #469 payments as well).
For method to be eligible all checks have to pass.
-Additionally metadata of all checks that didn't pass (or all checks and their status) should be present on "ShippingMethodQuote" object so appropriate feedback on why that particular method is not eligible can be shown to user.
e.g. extend ShippingMethodQuote object with properties:
eligible: boolean
and
eligibilityChecks: {id: ID, customFields: {{public custom fields/args of ShippingEligibilityChecker}}, eligible: boolean}[]