refactor: move sensorthings.entities to separate repository

[bertt]

we have duplicate code for sensorthings entities:

• https://github.com/gost/sensorthings-cli/blob/master/sti/cmd/entities.go
• https://github.com/gost/server/blob/master/sensorthings/entities/entity.go

solution: move entities code to separate repository

work involved:

• create new repository
• move code
• make gost/server dependent on new repository
• make gost/sensorthings-cli dependent on new repository
• test all the things

remarks:

• name: https://github.com/gost/core ?
• does the new repository need to have have dependency on gostErrors "github.com/gost/server/errors" or not? No dependency preferred (is only used once in https://github.com/gost/server/tree/master/sensorthings/entities).

[bertt]

created https://github.com/gost/core

[tebben]

dependencies removed from gost/server/sensorthings/entities ready for transfer to gost/core

[bertt]

entities copied to https://github.com/gost/core

[bertt]

entities code is removed from https://github.com/gost/server + dependency is added https://github.com/gost/server/commit/11906bf0c80f1cf707e56e21420961ef4c58f029

[bertt]

entities code removed from https://github.com/gost/sensorthings-cli + dependency is added https://github.com/gost/sensorthings-cli/commit/7ec1b29e786117474e2452e6b6e28aa38711aadd

[bertt]

todo: update connectors code too but thats a separate topic, closing for now