Added Seats Field to CourseRun Search API

[manan-memon]

Description:
This PR adds the seats object in response of Search API at search/course_runs/

Test Details:
Curl the following to get the test the response:

curl --location 'https://edlydev.discovery.multisitesdev.edly.io/api/v1/search/course_runs/' \
--header 'Authorization: JWT eyJhbGciOiJIUzI1NiJ9.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.SW6NLFjuuYEZFrPl3ZSsQ62Wacr_s0SjpU7_whZEiUQ' \
--header 'Cookie: sessionid=1|791hlyo9ct7qy8exhzv8jwkes7yfd419|xLk7QCOajwmw|ImY0MWJjMzg3MjA0NGFkZDNjMDAyZmUzNTc1YmJiOWI3MjFjMzAwNmE4Mzk1YzA2YzQ5ZTgxMzAzYmY4YzdhMjIi:1sOAO2:mWCkWrzx0nc4Z_ORI1fFR7grhLU; AWSALB=dL4PJgH1RJaQ1psVxjBq5r369Pumb9wcITwEu1UElQro67rI+CCOYL3qUYiFAw/dx288PhGwDVnuZNgHnrODsA3eLEkozQ2+epQKBlFvADUSWGc+CeuWxVa1NaU0; AWSALBCORS=dL4PJgH1RJaQ1psVxjBq5r369Pumb9wcITwEu1UElQro67rI+CCOYL3qUYiFAw/dx288PhGwDVnuZNgHnrODsA3eLEkozQ2+epQKBlFvADUSWGc+CeuWxVa1NaU0; sessionid=1|30ge1irho31r5o9jwnu6m1j0ojd9sx8o|JJ5OuPZP4W8L|IjdlYTRhOTY3YTY4NzZmMmFiNmU4ZDhiMWI2MDA4N2NhM2Y2NTI4ZTc3NjJjNTZmNmIzNjIzMjcxYTg1YmU4ZTYi:1soh4g:5sjilgKrjAU9x3rdbG2WJTejMfw; AWSALB=HMvI0HER7S4pK8tWuRQopjJjabx6AIFDuIGuxJ8G45zQoPck9LFfjZwtcyFO7S4M0WsAJYixzWUfITFnFVCAz0m+yfJtYwWR9cARjZD8VboeJcoI4zzhMGbK/mTn; AWSALBCORS=HMvI0HER7S4pK8tWuRQopjJjabx6AIFDuIGuxJ8G45zQoPck9LFfjZwtcyFO7S4M0WsAJYixzWUfITFnFVCAz0m+yfJtYwWR9cARjZD8VboeJcoI4zzhMGbK/mTn'

[Anas-hameed]

LGTM