Minor fixes to sync Plecos schemas with OEP-8 version 0.3

[ttmc]

These are minor fixes to bring the Plecos schema files more in sync with OEP-8 version 0.3. (There were some big changes in OEP-8 version 0.4, mainly adding the "services" array. Those changes aren't included in this issue.)

In plecos/plecos/schemas/metadata_local_v0_4.json

• Remove the overall checksum (not the file checksums). There should be no overall checksum in the local metadata because the overall checksum depends on the did and that is not known until later (after going through the register/publish flow).
• The price should have a "pattern" to restrict it to being digits, e.g ^[0-9]+$. I don't know if we want to allow scientific notation such as "1.3E10" or "1.23E+9". If yes, then the regular expression would be a bit different.
• In "links" - "items", "additionalProperties" is false but it should be true: OEP-8 gives marketplaces the freedom to add additional properties to link objects.

In plecos/plecos/schemas/metadata_remote_v0_4.json

• In file objects, the contentType is required (according to OEP-8).
• The overall checksum should be a string starting with "0x" and ending with 64 hex characters. Change the "pattern" regular expression from ^(.*)$ to ^0x[a-fA-F0-9]{64}$
• datePublished is required in the remote base object.
• See the local metadata task for the "pattern" on price (above).
• See the local metadata task for the "additionalProperties" of links (above).

More "patterns" (regular expressions) could be added. I just put some of the more important ones above.

[ttmc]

To construct and test regular expressions, I find https://regex101.com/ very helpful.