feat(core): add support for filters/scopes

[B4nan]

Allows to define filters that will be automatically applied to the conditions.
Filter can be defined at the entity level, dynamically via EM or in the ORM
Configuration.

@Entity()
@Filter({ name: 'expensive', cond: { price: { $gt: 1000 } } })
@Filter({ name: 'long', cond: { 'length(text)': { $gt: 10000 } } })
@Filter({ name: 'hasAuthor', cond: { author: { $ne: null } }, default: true })
@Filter({ name: 'writtenBy', cond: args => ({ author: { name: args.name } }) })
export class Book {
  ...
}

const books1 = await orm.em.find(Book, {}, {
  filters: ['long', 'expensive'],
});
const books2 = await orm.em.find(Book, {}, {
  filters: { hasAuthor: false, long: true, writtenBy: { name: 'God' } },
});

EM filter API:

// bound to entity, enabled by default
em.addFilter('writtenBy', args => ({ author: args.id }), Book);

// global, enabled by default, for all entities
em.addFilter('tenant', args => { ... });

// global, enabled by default, for only specified entities
em.addFilter('tenant', args => { ... }, [Author, Book]);
...

// set params (probably in some middleware)
em.setFilterParams('tenant', { tenantId: 123 });
em.setFilterParams('writtenBy', { id: 321 });

...

// usage
em.find(Book, {}); // same as `{ tenantId: 123 }`
em.find(Book, {}, { filters: ['writtenBy'] }); // same as `{ author: 321, tenantId: 123 }`
em.find(Book, {}, { filters: { tenant: false } }); // disabled tenant filter, so truly `{}`
em.find(Book, {}, { filters: false }); // disabled all filters, so truly `{}`

Closes #385