feat: add body serializer option

[Jak-Ch-ll]

Closes #91

As I said, this is really the most simple implementation for now. Just an optional bodySerializer option that allows modifying the body.

This is even settable as a global default when creating a custom client, but the types don't reflect that yet. This is because of how the options are typed in general right now. I want to look into that and might come around with some possible improvements.

Additionally, as described in #91, I really want to make the bodySerializer option type safe, so it's mandatory when it's needed. I think the trickiest part with that is the potential of a global default.

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[Norbiros]

1. Please run pnpm lint:fix to resolve linter issues
2. Could you please add a very simple test case for this feature?

Overall looks pretty good as first poc 👍🏻

[Jak-Ch-ll]

1. Please run pnpm lint:fix to resolve linter issues
2. Could you please add a very simple test case for this feature?

Overall looks pretty good as first poc 👍🏻

Both done, rebased into the existing commits ✅

Honestly I was looking for tests but I didn't expect them in the playground. Should have looked for the test script in the package.json 😅

FYI I originally had the tests in the runtime.spec.ts file in a separate describe block with its own setup call, but when I ran all tests together I got this error:

Error: No context is available. (Forgot calling setup or createContext?)
 ❯ useTestContext ../node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_playwrigh_995a05244efc9d5cba00fd3e85dfea15/node_modules/@nuxt/test-utils/dist/shared/test-utils.B8qEdk9k.mjs:53:11
     51|   recoverContextFromEnv();
     52|   if (!currentContext) {
     53|     throw new Error("No context is available. (Forgot calling setup or createContext?)");
       |           ^
     54|   }
     55|   return currentContext;
 ❯ loadFixture ../node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_playwrigh_995a05244efc9d5cba00fd3e85dfea15/node_modules/@nuxt/test-utils/dist/shared/test-utils.CT3RJOY3.mjs:79:29
 ❯ beforeAll ../node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_playwrigh_995a05244efc9d5cba00fd3e85dfea15/node_modules/@nuxt/test-utils/dist/shared/test-utils.CT3RJOY3.mjs:181:13

My solution was to create separate test files, which is probably a good idea anyway, depending on your taste in project structure.

The alternative would have been to have an outer describe block that calls setup once for all other describe blocks.