fix: Accept string payload in the Request constructor (#697)

- This is a follow-up to
#668 (comment).
- Let's accept `payload` as both str and bytes in the `Request.from_url`
constructor.
- The field `Request.payload` has a validator that accepts both. With
this change, we are consistent in the `from_url` constructor as well.
- We do a similar thing for the `Request.headers` as well.

src/crawlee/_request.py

@@ -183,7 +183,7 @@ def from_url(
         *,
         method: HttpMethod = 'GET',
         headers: HttpHeaders | dict[str, str] | None = None,
-        payload: HttpPayload | None = None,
+        payload: HttpPayload | str | None = None,
         label: str | None = None,
         unique_key: str | None = None,
         id: str | None = None,
@@ -195,6 +195,9 @@ def from_url(
         if isinstance(headers, dict) or headers is None:
             headers = HttpHeaders(headers or {})
 
+        if isinstance(payload, str):
+            payload = payload.encode()
+
         unique_key = unique_key or compute_unique_key(
             url,
             method=method,
@@ -276,7 +279,7 @@ def from_url(
         *,
         method: HttpMethod = 'GET',
         headers: HttpHeaders | dict[str, str] | None = None,
-        payload: HttpPayload | None = None,
+        payload: HttpPayload | str | None = None,
         label: str | None = None,
         unique_key: str | None = None,
         id: str | None = None,
@@ -317,6 +320,9 @@ def from_url(
         if isinstance(headers, dict) or headers is None:
             headers = HttpHeaders(headers or {})
 
+        if isinstance(payload, str):
+            payload = payload.encode()
+
         unique_key = unique_key or compute_unique_key(
             url,
             method=method,